Magnetic sensors are employed in a wide range of technological applications including automotive, industrial and consumer applications. These applications need magnetic sensors that operate reliably and take accurate measurements.
Typically, magnetic sensors include one layer of magnetic film in which the orientation of magnetization is alterable and another layer of magnetic film in which the orientation of magnetization is fixed or “pinned” in a particular direction. The magnetic film having alterable magnetization is referred to as a sense layer and the magnetic film that is fixed is referred to as a reference layer. The resistance through the magnetic sensor differs according to the orientation of magnetization in the sense layer and the reference layer.
Typically, magnetic sensors based on the tunneling magneto-resistive (TMR) effect or the giant magneto-resistive (GMR) effect use two magnetic orientations. Sometimes, the magnetic orientation of the sense layer without an external magnetic field is aligned along the length of the magnetic sensor, in a long narrow stripe, and the magnetic orientation of the reference layer is pinned via exchange coupling. Within an external magnetic field range, the resistance through the sensor stripe changes almost linearly. However, for some magnetic sensors, such as TMR sensors, shape anisotropy cannot always be used successfully.
Sometimes, one exchange coupling is used to pin the magnetic orientation of the reference layer and another exchange coupling is used to achieve a preferred magnetic orientation in the sense layer. An exchange coupling is achieved via heating a magnetic sensor above a blocking temperature in an applied magnetic field and subsequent cooling of the magnetic sensor within the magnetic field, which is also referred to as tempering the sensor. The direction of the applied magnetic field is inscribed in the magnetic layer(s). To create a linear characteristic, the magnetic orientations of the sense layer and the reference layer are rotated by 90 or 270 degrees.
When using two exchange couplings, two blocking temperatures are used to achieve the different magnetic orientations. Initially, an exchange bias is inscribed via the higher blocking temperature, which inscribes the magnetic orientation of the applied magnetic field in the system with the lower blocking temperature and the system with the higher blocking temperature. In a second tempering step, an exchange bias is inscribed in the system with the lower blocking temperature via heating the magnetic sensor to a temperature between the higher and lower blocking temperatures. However, sometimes this second tempering step causes degradation in the magnetic sensor, such as by diffusion or corrosion. Also, sometimes the second tempering step affects the system with the higher blocking temperature, where the magnetic orientation could be changed. In addition, using a system with a lower blocking temperature has negative effects on the stability of the magnetic sensor.
For these and other reasons, there is a need for the present invention.